text:
Do you need 40 hours in a helicopter to get a PPL add-on if you already have an ASEL PPL?
FAR 61.109(c) says:

For a helicopter rating. Except as provided in paragraph (k) of this section, a person who applies for a private pilot certificate with rotorcraft category and helicopter class rating must log at least 40 hours of flight time that includes at least 20 hours of flight training from an authorized instructor and 10 hours of solo flight training in the areas of operation listed in §61.107(b)(3) of this part, and the training must include at least...

The question is, does the 40 hours have to be in a helicopter, or can hours in ASEL meet this requirement? All the training centers seem to be saying it has to be 40 hours in helicopter. Can you theoretically fulfill the other requirements inside the 20 hours training and 10 hours solo in heli and be compliant with 61.109(c) with only 30 hours in heli?
Actually, it's not clear to me if the 20 hours of dual has to be in helicopter.

Considering the difference between legal and smart, you could nonetheless potentially satisfy the training requirements with just a few hours in a helicopter. FAR 61.109 specifically notes that certain requirements must be satisfied in a helicopter:

61.109(c)(1): "3 hours of cross-country flight training in a helicopter;"
61.109(c)(2): "[...] 3 hours of night flight training in a helicopter that includes" a 50nm flight and 10 takeoffs and
  landings
61.109(c)(3): "3 hours of flight training with an authorized instructor in a helicopter in preparation for the practical test..."
61.109(c)(4): "10 hours of solo flight time in a helicopter..."

Theoretically, the flight training could be all combined, resulting in 3 dual and 10 solo hours in a helicopter, the rest of the requirements being fulfilled in a different category or class of aircraft.
The FAA issued an interpretation (O'Mara, 2012) about these rules in 2012. The discussion was about weight-shift-control aircraft (61.109(j)), but the reasoning is applicable to other sections of 61.109:

In your letter you indicate that an examiner you spoke with stated that all of the aeronautical experience requirements specified in § 61.109(j), to include the 40 hours of flight time, 20 hours of flight training, and 10 hours of solo flight training must be completed in a weight-shift-control aircraft. That statement was incorrect.
[...]
...[T]he requirement for an applicant for a weight-shift-control aircraft to have 40 hours of flight time may be met by obtaining flight time in any category and class of aircraft.
Paragraphs (j)(1) through (j)(5) of § 61.109 specify the flight training that must be received by an applicant to receive a weight-shift-control aircraft rating. Flight training is defined in § 61.1 as "training, other than ground training, received from an authorized instructor in flight in an aircraft." Similar to the definition of flight time, it also does not include a limitation that the training be obtained in any particular category and class of aircraft.

That said, this is somewhat academic: in order to get a solo signoff from a CFI, a student would at least require nontrivial training time in a helicopter. I'll also cite another interpretation (Zomnir, 2010) I came across in my research, which has perhaps the most obvious statement in an FAA legal interpretation:

First, you ask whether your student is required to be proficient to the practical test standards to pass a practical exam. The answer is yes.

The Zomnir interpretation link is 404ing, so I can't check. Did someone really ask the FAA if a student is required to be proficient to the practical test standards to pass a practical exam‽

There is a very common misconception about the required hours to complete a rotorcraft add-on. Call any helicopter school (or look it up online) and they will tell you it is 30 hours if you have a PPL.
Here is the breakout:
14 CFR 61.109 in part states:

(c) For a helicopter rating. Except as provided in paragraph (k) of this
      section, a person who applies for a private pilot certificate with
      rotorcraft category and helicopter class rating must log at least 40 hours
      of flight time that includes at least 20 hours of flight training from an
      authorized instructor and 10 hours of solo flight training in the areas of
      operation listed in §61.107(b)(3) of this part, and the training must
      include at least—

The operative section is this one:

...includes at least 20 hours of flight training from an authorized instructor
      and 10 hours of solo flight training in the areas of operation listed in
      §61.107(b)(3) of this part...

If you read 14 CFR 61.107(b)(3), you will see that you cannot complete the 20 hours with an authorized instructor or the 10 hours solo in anything but a rotorcraft:

(3) For a rotorcraft category rating with a helicopter class rating:
  (i) Preflight preparation;
  (ii) Preflight procedures;
  (iii) Airport and heliport operations;
  (iv) Hovering maneuvers;
  (v) Takeoffs, landings, and go-arounds;
  (vi) Performance maneuvers;
  (vii) Navigation;
  (viii) Emergency operations;
  (ix) Night operations, except as provided in § 61.110 of this part; and
  (x) Postflight procedures.

14 CFR 61.107(b)(3) "in the areas of operation" logic applied to this requirement could be read to apply just to the 10 solo hours, or include the 20 dual training hours, or to all 40 hours of flight time. It seems like you would either have to support either the 10 or 40 hour answers; 10+20 seems like an inconsistent halfway measure.

@pilothead I am not sure why you think it is inconsistent, IMHO 61.109 is very clear. Additionally, I did a rotor addon many years ago and it was 30 hours when I did it. Just a cursory search on Goole here, here, here and here all verify the 30 hour requirement for the addon rating.

30 hours requires the 69.109(c) wording "includes" to be read differently than "and" as neither has "areas of operation" as a direct modifier. If this were clear there would be no need for this question and of course helicopter flight schools would take the position that 20 hours of dual has to be in a helicopter.

I guess I am missing your point....are you are saying that all of the operations in 61.107(b)(3) can be done solo since they used the word "and" and all of the helicopter flight schools have created a conspiracy to force everyone to do 20 hours dual in a helicopter? :-)

Nathan's answer clarifies that the FAA in O'Mara considers the 40hrs flight time and 20hrs flight training acceptable in any aircraft, and the 10hrs specific to helicopters, subject to subparts 1-4. The reference is to 61.109j weight shift rating, but the wording is identical to the 61.109c helicopter rating requirement.

To start, 61.109j is not identical to 61.109c, look at the landing requirements. Second, if you add up all the times it says "helicopter" in 61.109c you get more than 10 hours. Our local FSDO said 30 hours when queried and they also made another good point: IACRA won't let you do a rotorcraft add-on with less than 30 hours in a rotorcraft. Not saying that IARCA or the FSDO is 100% correct, but every single helicopter flight school I looked up uses the 30-hour rule as well. Are they all wrong along with the FSDO and IACRA? If so, I am shocked that no one has taken them to task!

Yes it needs to be in a helicopter.

§61.107   Flight proficiency. (a) General. A person who applies for a
  private pilot certificate must receive and log ground and flight
  training from an authorized instructor on the areas of operation of
  this section that apply to the aircraft category and class rating
  sought.

You should note that the wording in §61.109(c) "must log at least 40 hours of flight time that includes at least 20 hours of flight training from an authorized instructor and 10 hours of solo flight training in" is identical to the wording in §61.109(a) for airplane category.
The interpretation is you need 40 hours total with a minimum 20 hrs dual and a minimum 10 hrs solo.  The other 10 hrs can be either dual or solo.
You should note the requirements of §61.63 (b) for additional ratings. Basically, the only thing you're exempted from the private pilot knowledge test.

§61.63   Additional aircraft ratings (other than for ratings at the
  airline transport pilot certification level). (a) General. For an
  additional aircraft rating on a pilot certificate, other than for an
  airline transport pilot certificate, a person must meet the
  requirements of this section appropriate to the additional aircraft
  rating sought.
(b) Additional aircraft category rating. A person who applies to add a
  category rating to a pilot certificate:
(1) Must complete the training and have the applicable aeronautical
  experience.
(2) Must have a logbook or training record endorsement from an
  authorized instructor attesting that the person was found competent in
  the appropriate aeronautical knowledge areas and proficient in the
  appropriate areas of operation.
(3) Must pass the practical test.
(4) Need not take an additional knowledge test, provided the applicant
  holds an airplane, rotorcraft, powered-lift, weight-shift-control
  aircraft, powered parachute, or airship rating at that pilot
  certificate level.

it says "40 hours of flight time". It does not stipulate that it must be in category from what I can see. It only says that some of that time must include the specific training described later.


text:
Is there a difference in landing fees for charter and scheduled flights?
Is there a qualitative difference for landing fees/charges between charter and scheduled flights at a particular airport?

Please tell me which particular airport your are interested in. Otherwise answer would be maybe. Schedule flight maybe able to deal some favourable landing fees at a particular airport.

This is almost completely dependent on the airport but we can talk somewhat generally.
For what its worth airlines (scheduled carriers) can often negotiate lower landing fees by being able to guarantee a specific amount of flights into an airport. Charter companies don't have this luxury since it can be hard for them to predict where and when they are going. On the other hand some fees are set by the local government, for example here in Philadelphia any airport in the city (KPHL and KPNE) has landing fees set by the city. You can find the official legislation here and I assume thats non negotiable but I cant be sure.
There is also a bit of a discrepancy due to the type of planes. Landing fees are often charged by weight and in most cases charter planes are flying smaller stuff than the scheduled carriers and their fees may be lower as a result (even though the scheduled carrier has a lower rate). If we look again at Philadelphia we can see that they have fees for both weight and passengers which generally go hand in hand however a charter company may be flying a big RJ for just one guy while a scheduled carrier may be operating the same plane at its maximum seating arrangement.
Charter carriers may also have the luxury of flying into smaller satellite airports where the fees may be lower or structured differently. Since some of these airports could potentially be private or private/public they have the ability to charge what they chose. Some will even have no fee if you buy a pre prescribed amount of fuel.
For other points of reference you can check out the

New York Landing Fees
Miami Landing Fees

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sort: votes


text:
Can any conclusions about aerodynamic efficiency be drawn on the basis of fly-by noise?
Edit: I'm beginning to think this is more suited to the physics stack exchange, but I'll keep this here for a while...
Being bored mindless, surfing the interwebz, I came across this video, where a Sukhoi SU-57 is performing aerobatics:
Youtube: Су-57 с новым звуком пилотаж на форуме Армия 2020 Кубинка
At timestamp 1:45 the plane makes a pass, emitting a distinct screech with prominent components at about 1000, 2500 and 3600Hz.
This screech may very well be from external "add on" equipment such as radar reflectors (see this Q&A), as I kind of combine such noise to some kind of "fault" in the aerodynamics, and therefore I think during design phase sources of such disturbances in airflow would be eliminated from the airframe.
As first hand observations from real life, two examples: sailplanes and the B757, close encounter of which I have found to be memorable: the sailplanes for being very quiet, emitting a slight whisper with maybe a dash of a faint whistle at a high speed fly-by, and the B757s making a very distinct and sharp hizz on short finals (B757 is categorized as "heavy", not due to its size or mass, but for the nasty wake turbulence it makes).
So: can one draw conclusions about planes aerodynamic efficiency based on acoustic observations, and if so, to what extent?

Any aerodynamic noise will be drowned out by engine noise.

Ummm... no. I have very different experiences. Yours may differ, of course.

What is your experience hearing a B757 in flight with both engines shut down? Even at idle, they produce a staggering amount of noise.

To answer your question, none. But: many a times have I observed a plethora of jets, large and small from tens to tens of thousands of feet away. I'm no stranger to seats of them either, and behold, my ears can separate noises from engine and, say, fully extended flaps; a fighter jet performing a full g pull makes distinctly different sound than one travelling at same speed and configuration at 1g. Now: I'm not going to argue about this with you no more.  To back off a little: I can't actually hear nothing but the engine noise of a jet at cruising altitude if I'm not on the plane myself

Invariably, aerodynamic efficiency improves with newer designs and so it was from 747-200/300 (not glass cockpit) to 747-400 (winglets + increased span + glass cockpit) to B777 to B787, from A300 to A350. Industry wide pilots were never disappointed by this and proud to be flying the latest lowest drag 'slippery' airplane.
But using noise as an indication of aerodynamic efficiency, could have peculiar factors come into play. The ultra quiet and ultra efficient B787 is so 'clean' or 'slippery' that it requires great anticipation and confidence in technique to fly both the 'speed control' and the CDA Continuous Descent Approach as published at some very major airports.
Many a time, it cannot be flown without the use of speed brakes, the use of which, is (was?) anathema for many pilots.
The  normal expectation is that as long as the speed is above the selected or target speed with speed brakes deployed to aid in reducing the speed, and especially during descent/ the engines would be at flight-idle. Contrary to this, a design feature of the autoflight system pushes the thrust levers up so that they are not at idle but producing thrust when the pilot is doing her best to make the airplane decelerate! So, earlier generation airplanes would have thrust at idle at these points, but the 787 does not.
Even under these circumstances it probably has a smaller noise footprint but an inability to decelerate as per the ATC published speed control targets, could for eg result in gear being lowered earlier than the published preference and this all adds to the sound. The Gear extension point is actually being monitored for variations at such airports.
Despite the above information, theoretically, aerodynamic efficiency could certainly be gauged on the basis of noise provided you were to compare like to like - the comparison would have to be between aircraft similar category, all the airplanes would have to be doing the same maneuver in equivalent configuration eg.

passing 500ft on final approach, max landing weight, landing config with gear down, on speed, and stabilised on approach

OR

level fly by in clean config, height 400ft, 270kts speed.

And there's no accounting for what factors, apart from aerodynamics and engines maybe discovered to be contributing to the noise level.

Thank you for your answer, I have not had the pleasure of riding 787, but A350 is quite quiet too, and slippery as well. Regarding approach procedures, they are not set in stone, and CDAs will be adjusted as the planes "evolve". And I agree, feels criminal CDAs are flown with speedbrakes open, partially defeats the purpose, but I think it is economically and -logically better the CDA cuts fuel consumption of the less efficient planes rather than the more efficient ones. More gets saved that way.

I've modded the answer a bit, thank you.

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text:
Regaining airworthiness of a commercial aircraft
If a large commercial part 121 aircraft has lost it's airworthiness, and remained in that state for several months while being operated despite the loss of the airworthiness, what inspections and documentation are needed to make this airliner airworthy again per FAA or EASA rules?
For clarity, let us assume the airworthiness is lost due to lack of required maintenance, and the period of operation after the loss of airworthiness is not fully reliably documented.
Edit: By coincidence, I am now able to provide you with a real life example of such a situation:
Bermuda revokes licences of over half of Russia's airline fleet

“International sanctions on the aviation sector have had a significant impact on the ability to sustain safety oversight on Russian operated aircraft on the Bermuda Aircraft Registry. The airworthiness system has been restricted to the point that the Bermuda Civil Aviation Authority is unable to confidently approve these aircraft as being airworthy.”

I'm not sure what you mean by "losing" an airworthiness certificate? They don't expire, so are you asking about a scenario where the FAA took action to revoke it? If it wasn't revoked then you'd just need to get the maintenance done.

@Pondlife Standby, let me get my terminology checked...

When you say “lack of required maintenance”  are you thinking about an AD that hasn’t been complied with or just an annual or 50/100 hr (or whatever period part 121 requires) inspection? Or something else?

@Jim please see the edit I just made.

Airworthiness is a matter of passing inspections to ensure the aircraft still complies with its original Airworthiness Certificate and any Airworthiness Directives that have been issued since then.
If you’re doing the necessary maintenance to keep a plane safe for flight, it’s a relatively minor thing to do the inspections too and keep it airworthy. In fact, you’re probably discovering much of the need for said maintenance as part of your inspection program in the first place.
If you can’t perform the maintenance needed to pass an inspection, whether that’s due to a lack of parts, labor or cash, then the plane should be grounded until you can.
It doesn’t matter whether the aircraft is grounded for a day or a decade; as soon as it can pass the appropriate inspections, it’s airworthy again as if nothing had happened.

Thank you for your answer. The point here is, that the aircraft remains in use despite lack of proper maintenance, and the documentation of this continued operation may be questionable. I'd give an example, but my original question about this was deemed opinion based when I did so...

@Jpe61 Flying an unairworthy aircraft doesn’t change what has to be done to make it airworthy again.

it does in the sense that operating the plane under obscure conditions makes it harder to determine what needs to be done to regain airworthiness. "all" is obviously not an answer, as that would be quite an ordeal

@Jpe61 It’s still the same inspection. Flying in the meantime may add some extra wear and tear that needs more repairs, but not doing the required maintenance means your planes may stop flying anyway—and rather abruptly.

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text:
What are good apps to help with VFR navigation on my first solo?
Currently I am a student pilot in Spain (Barcelona) and I am 2 days away from my solo. I wanted to ask if there is any good application that will help me on VFR navigation since I'm allowed to go to all Barcelona in my solo, but I am afraid if I get lost I see my self weak in VFR flights and specially to locate my position so any good app in the iPhone device that will help me in navigation because I want to visit the whole Barcelona and not just staying at places that I know because I will get bored for sure.

If you get lost then I can guarantee you will immediately stop being bored :-) Seriously, this is a question for your flight instructor. He should only allow you to fly in conditions that you're properly trained and competent in, and if you are weak in navigation then you should fix that first with him. An app by itself won't help you navigate any better unless you know how to use it.

lol yeah i can manage that but i want to make sure i'm safe you know what i mean that's why i asked for some apps that will help me on my nav

If you can't be safe without an app, then you are not ready to do what you are planning to do.  You should tell this to your instructor.  Your first solo is not a time to go sight seeing, and if you think your first solo will be boring, then you should think about why you want to fly.

@Pondlife  I would argue that even being able to use the app will not fix underlying weaknesses in aviate, navigate, communicate.

"I want to visit the whole Barcelona" - Don't be this guy

@DanHulme yeah man what i can do 50 hours solo at least i can do something that makes the time pass quickly. or you want me to stay doing circuits for the whole 50 hours lol

Just because you do something on your first solo doesn't mean you have to do only the exact same thing every other time. Why would you want to put yourself in an unfamiliar situation with an unfamiliar app your first time?

There is a difference between flying solo and your first solo. Which are you asking about? Your first solo should ONLY be circuits. If your instructor tells you different, then ask why. If you are asking about solo flying before you qualify, you should be only be doing flights you have planned 100%. Do not use an app, or start random sight seeing until at least you have passed your exam.  Learn to fly with a map, a watch and a compass first.  Then think about GPS and apps.  "Plan the fly" and "fly the plan".  Too many dead pilots didn't listen to advice.  Please don't be one of them.

Does your aircraft have a GPS installed? All you would need then is to know how to push the Direct-To button and follow a heading.

What will you do if the battery on whatever device you are using suddenly goes flat? You shouldn't even think about leaving the circuit and training area by yourself until you have been taught to properly navigate. But perhaps you should make it clearer that you are asking about flight planning and navigation apps, because they do exist.

@Ben i'm not that stupid Ben to depend only on my Phone, i will plan everything as normal procedures is saying but i will keep the phone as a backup not as the main navigation just to track my progress and my performance

I'm not sure I understand the downvote here. This question is based on what a pilot is planning to do. While the title question is too broad, he narrows it down in the body. His plan, as stated, may be a bad one, but that doesn't make the question bad.

Even without a GPS, all sorts of things can happen to maps and to compasses. Are all the critics sure that they are able to navigate without map and compass? Also, in the US, we are taught the lost art of lost procedures: climb, communicate, confess, comply, as well as how to call PAN PAN/MAYDAY on 121.5, and how to ask for "DF" (direction finding) from Flight Service. Perception of OPs aeronautical decision-making, I laud him for asking the question before getting in the aircraft.

If he be asking if there were something for the iPhone in case all else fails, there would be many answers. Even Google maps would suffice. When the question is for something to help in navigation, there is nothing. If every navigation system died. i'd whip out my iphone or ipad and use Personal Black Box for emergency use but I would not rely on that in an a/c if I had a single VOR functioning.

No, there is no "app" you should be seeking at this stage of your training to do what you're asking about.
First, if you have questions about what you'll be doing while you're flying solo and how you'll be navigating, you should be discussing this with your instructor, not on an internet message board.
Second, whatever tools you'll use in flight need to be ones that you've used under supervision of an instructor, so that you understand how to use them, and what their limitations are.  Student solo flying is NOT the time to go exploring technology and apps you've not used before, in addition to the full-time tasks of maintaining aircraft control, aviating, navigating, and communicating.  Too many dead pilots got focused on some technological toy and, without realizing it, lost situational awareness and/or control of their aircraft.
Third, before you are navigating anywhere solo, you need to be able to (and demonstrate to your instructor that you can) navigate successfully with the tools you have -- a map, compass, stopwatch, and the pre-flight planning you've done.  If you don't feel up to that task (which has been all of us early on in our training -- nobody is born with a built-in INS), work on those skills with an instructor.  Navigation apps are fine, but they supplement, rather than substitute for, your own skills when it comes to VFR navigation.
Fourth, the fact that you would come here to pose such a question, rather than to your instructor, is troublesome: you want an app that can enable you to go off on adventures sightseeing all of Barcelona, but you aren't discussing these plans with your instructor... perhaps he might not approve of all that you're planning?  Perhaps, maybe, you should limit your expectations of how much adventuring & exploring you'll undertake initially, and have a more complete discussion with your instructor of what you are, and are not, expected to do when, or if, you are signed off to solo.  (As stated in comments, the initial solo is typically pattern work only, rather than navigation.)
Flying is about far more than aircraft control; it is a matter of judgement as well, and the difference between sound judgement and the lack of same can make the difference between a successful or a catastrophic outcome of a flight, at least as much as the difference between great and mediocre "stick & rudder" skills.

Please don't do something that you are not used to do at your first solo. First solo is always something you are waiting for, may be from the first day of your flying. So,I hope, it will not be that boring. It is not a good idea at all to depend upon a app when you are flying your 1st VFR solo. Please take enough preparation considering the terrain around your flying areas/routes. Search in Google map/earth to be very specific. Take enough preparation on the use of radio aids and diversion procedures. Be confident about emergencies. And, enjoy your solo. Good luck!!! Have a safe flight.

yep you are totaly right i agree with you but what i mean is for my others solo not the first one of course, i will prepare everything as you sais of course! just i tought that it will be always good to use a backup to track my progress and make sure i'm in the right path this why i asked for a navigation app maybe some of the Private Pilot are using them to choose the best one. Thank you :)

Google Maps is not a flight planning tool. To check the terrain clearances and controlled airspaces you need to use the map issued by your country's civil aviation authority. This will probably be a paper map in early navigation flights.

Yes Google map is not a flight planning tool.. but u should understand that in charts only the obstacles are maked with height.. you can check those in google to have visual understanding.. Thats what I did mean @ Dan Hulme

In a solo, you shouldn't be doing anything you haven't already done with an instructor onboard, but without his assistance. If you haven't navigated the trip you plan to fly solo yet without your instructor doing the navigation for you, then you aren't ready to solo.

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text:
How is a conflict between aircraft with TCAS I and II resolved?
Let's say there are two aircraft in a risk of collision, one of them is equipped with Traffic Collision Avoidance System (TCAS) I so it will get only a Traffic Advisory (TA), however the second one is equipped  with TCAS II, and it will get both TA and Resolution Advisory (RA). What is the procedure to be followed if there is a risk of collision, the TCAS II aircraft must follow the RA, and the TCAS I must follow the ATC instructions or what is the procedure in this case?

TCAS 1 will only give a Traffic Advisory (TA). The crew will lookout for the other aircraft and take evasive action if necessary. They also may contact ATC for instructions. They will follow ATC instructions unless they see that the instructions will bring them into the path of the other aircraft.
TCAS II will give a traffic advisory first, then a Resolution Advisory (RA). The crew has to follow the resolution advisory and inform ATC of their maneuvering (e.g. TCAS climb, Big Jet 123). They are trained not to follow ATC instructions that oppose the TCAS RA.
Note that TCAS II RA's are always in the vertical plane; TCAS II can not give instructions to turn.

According to AC 120-55C, the TCAS II aircraft will issue a TA (Traffic Advisory) and an RA (Resolution Advisory) if necessary and the pilot should follow any RA immediately and safely.  The TCAS I aircraft should attempt to establish visual contact and maintain a safe distance from the other aircraft while following good operating procedures and ATC clearance. TCAS I only offers information about the traffic around the aircraft and does not give RA's.  Meanwhile the TCAS II aircraft will see the TCAS I aircraft but will not expect the intruder to follow RA's or coordinate TCAS behavior, similar to if the TCAS I aircraft had only a mode C transponder or had TCAS II but with "TA only" active.  See the table below for response details:

Table Source
The behavior on the TCAS II aircraft is not all that different than if both were TCAS II equipped and the aircraft in question was the first to issue a Resolution Advisory (RA).   The first aircraft will select an RA direction (up or down) that gives the most vertical separation from the intruder.  The RA direction and strength for the first aircraft is calculated independently of the expected TCAS response of the intruder, except in a few exceptional cases.
You will not get coordinated behavior if one of aircraft is TCAS I equipped, but coordination only applies when the other aircraft has already issued an RA or an RA reversal is necessary.  To give a brief description of coordination, normally the two TCAS II aircraft issue RA's at a slightly different time, and the first to declare an RA sends an "intent message" to the other to prevent it from issuing a contradictory RA. Coordination will also send a message during an RA reversal indicating that the other plane should follow the reversal. The exact details of coordination is beyond the scope of this question.
TCAS II and TCAS I will also have slightly different requirements for displaying a TA.  For example, the logic was adjusted in TCAS II v7.0 to reduce thresholds for RVSM airspace.  So the TA may be displayed on one plane when it is not displayed on the other.
It is important as well to remember the cardinal rules of TCAS:  Always immediately and safely follow an RA (except in cases of stall or terrain), but do not deviate from ATC clearance and good operating procedures just to avoid a TA.    The Uberlingen mid-air collision underscored the importance of following an RA and not ATC.  The crash was caused by one plane following the RA and the other following conflicting ATC guidance. See AC 120-55C for more details on expected pilot response to TCAS.
TCAS logic is complicated when you get into the inner workings.  I'd recommend reading the FAA's guide for more details.  Most of this information comes from that booklet.


text:
Hobbs vs Tach time?
Other posts didn't answer my question. In A&P school months ago they told us the difference and can't find where I wrote this down. One of them was used to find engine time and the other was used for aircraft total time.

I wrote in my post that it didn't answer what my question was.

Then you will need to clarify how it doesn't answer your question. The answer explains that one is based on engine revolutions and the other is based on regular time, which should explain which relates to engine time and which relates to aircraft time.

Generally the engine time is measured using the Tach (and is what the engine is serviced based on) while the airframe time is measured off the hobbs meter. This is based on the idea that the hobbs is running when then plane is "on" (usually when oil pressure is present) but the tach time measures engine wear time. In theory the engine wears slower at a lower RPM.
As for maintenance this should be elaborated on a bit since most people actually measure this incorrectly sometimes. Tach time is always running however it does not always run at a 1 Tach Hour = 1 Clock Hour rate. Generally the tach timer is set up to run 1 Tach Hour = 1 Clock Hour at 70% power (or some other often used cruise power). Any RPMs below that will run the tach timer slower and any above it will run it faster. Tach time is generally accepted for engine service since it directly measures engine time (it actually measures RPM count) but for service on the rest of the airframe there are a few options. When it comes to a Hobbs meter there are various ways they can be connected. I was always taught it was based on oil pressure however some planes seem to have it wired to the master switch and more importantly others are wired to a squat switch on the gear. The FAA defines Time In Service as

Time in service, with respect to maintenance time records, means the
  time from the moment an aircraft leaves the surface of the earth until
  it touches it at the next point of landing.

Lets take the 100 hour inspection which is required after 100 hours of Time In Service

(b) Except as provided in paragraph (c) of this section, no person may
  operate an aircraft carrying any person (other than a crewmember) for
  hire, and no person may give flight instruction for hire in an
  aircraft which that person provides, unless within the preceding 100
  hours of time in service the aircraft has received an annual or
  100-hour inspection and been approved for return to service in
  accordance with part 43 of this chapter or has received an inspection
  for the issuance of an airworthiness certificate in accordance with
  part 21 of this chapter. The 100-hour limitation may be exceeded by
  not more than 10 hours while en route to reach a place where the
  inspection can be done. The excess time used to reach a place where
  the inspection can be done must be included in computing the next 100
  hours of time in service.

As such a hobbs meter connected to a squat switch would actually be the most representative of Time In Service which is when hourly maintenance needs to be performed.

-1. All maintenance (airframe & engine & else) is based on flight time, not engine run time. For single engine aircraft (and many twins) both airframe and engine (and all other components) times are measured by tach time. Hobbs time starts ticking when the engine is started. Tach time starts ticking at high RPMs (such as at take-off). Since the FAA bases maintenance on flight time, the tach time is a more accurate representation of flight time.


text:
Is it possible to fully utilise excess heat from an aircraft to power other systems required for the aircraft to function?
Is it possible to fully utilize excess heat from an aircraft engines to power other systems required for the aircraft to function, using heat exchangers and other clever engineering tools and techniques? This would seem to me to amount to massive energy efficiency and fuel savings.

Is the pilot included in "other systems needed for the aircraft to function" because the cabin heat is exactly like that in my little plane :)

Harvesting heat for electricity is not very efficient.

In a sense, bleed air from jet engines is using this technique.

And massive weight and cost increase.

"This would amount to massive energy efficiency and fuel savings.", No it really would not be.  The Avionics, lights, hydraulics, environmental controls really do not draw very much energy compared to the power needed to keep the plane flying.  The additional weight of a heat-capture/convert system would be a far bigger factor.

On this board we obey the laws of thermodynamics.

There are heat engines that convert heat into mechanical energy, which can drive useful things around the aircraft. Examples that actually drive systems that we use are

The steam turbine used for electrical power generation in remote area's, driven by the waste heat from gas turbines.
The stirling engine, used for driving submarines due to it being whisper quiet.
The classical steam engine, also very quiet.

All these engines come at a weight penalty of course. It makes a lot of sense to add a steam turbine to a gas turbine in a power station; adding a steam turbine next to the jet engine is less clearly beneficial. It has not been done.
So yes some of the waste heat of the aircraft engines can be used. But thermodynamics says that never all heat can be transformed - the theoretical maximum efficiency is a function of the maximum and minimum temperatures. Only a heat engine with a perfect Carnot cycle, a heatsink of 0 K or an operating temperature of $\infty$ K and no heat loss other than through the engine process will transform all heat. And we cannot make that in an environment that is neither 0K nor $\infty$ K.

Nitpick: You need a cold side of 0K or an infinite operating temperature, not both. (1-Tc/Th). More practically, the temperature differential is dictated by the environment (outside air, waste heat) and not an engineering choice.

@MSalters right-o, have incorporated.


text:
What is the preferred way to slow down an airliner on a long runway?
EDIT: Another way to phrase the question is, given an extra long runway, does it make sense to substitute mechanical braking (which is the primary way to slow down in a typical situation) partially or completely with something else, to reduce brake wear / roll down the runway faster?

Consider an airliner landing on a very long runway (e.g. Boeing 737 landing on 12,000 feet+). The pilots are planning to vacate the runway near the end, so there is a lot of distance to spare.
Ignore noise procedures, and assume there is no traffic behind to suggest an early exit. What would be the preferred way to slow down the aircraft in this scenario?

Light application of reverse thrust only, no brakes?
Light application of brakes only, no reverse thrust?
Moderate braking until below a certain speed (e.g. 80 knots), then "coast" the airplane down the runway?

(This question addresses the primary means to slow down after a typical landing, while this question is about an idealistic case where there is plenty of braking capacity to spare.)

I feel the answer to question you link to answers this question. It states the breaks are always the primary method. Using air from the engine is always undesirable from an economic view as it's using fuel for something other than thrust for flying.

@notts90 I can't see why the linked question is a duplicate. The question is different and the answers are not applicable.

There are several ways to slow down an airliner:

aerodynamic drag
friction in the wheel bearings
reverse thrust
wheel brakes

and a few even used a brake chute, but that went out of fashion a long time ago. If the runway is inclined, landing uphill also will slow down the aircraft.
Friction and drag you get for free, so I would rely on those first. Since they are highest when the speed is high, use them first until the aircraft is so far down the runway that it would overshoot without additional application of wheel brakes. Use spoilers and leave the flaps down to maximise the deceleration. Only at the end add wheel brakes so you have slowed down to taxi speed when the exit of the runway is reached.
So the best answer is none of the above but:

Coast along with flaps and spoilers out until you need to brake more to avoid an overshoot. Then apply wheel brakes.

The exact way how brakes should be applied depends on their type.

So no reverse thrust ?

@Antzi: No, that needs fuel and makes noise. Brakes are fine. The key is to take as much energy out of the plane as possible without using either reversers or brakes, and to use brakes for the remainder.

@Antzi reversers as needed to slow down, but if you can do so without reversers, do so without reversers. Saves fuel, and might even be required for noise abatement reasons.

@PeterKämpf Deploying the reversers without actually spooling up the engines also provides a non-negligible amount of force with little or no extra cost, too, right?

@reirab: Yes, particularly at higher speed. Maybe it is worth to deploy them right after touchdown but leave the engines at idle.

Maybe we can also add the slats here?

@Peter, on most aircraft that have them, slats simply come out with the first position of flaps, so there is not much point in mentioning them. They also don't generate that much drag, just increase the critical angle of attack to compensate the decrease due to flaps.

To make it efficiently its better to start with thrust reversers to reduce speed just after touchdown. As speed reduces the effect of the reversers reduce. To avoid FOD(foreign object damage) risks it is not recommended to use high revers thrust with low ground speed.
This what Airbus says and advises to the flight crews of A330s.
Three systems are involved in braking once the aircraft is on the ground:
• The ground spoilers
• The thrust reversers
• The wheel brakes
THE GROUND SPOILERS
The ground spoilers contribute to aircraft deceleration by increasing aerodynamic drag at high speed. Wheel braking efficiency is improved due to the increased load on the wheels.
REVERSE THRUST EFFICIENCY
Thrust reversers are more efficient at high speeds.
Below 70 kt, thrust reversers efficiency rapidly decreases. Below 60 kt with REV MAX selected,engine stall may occur. Therefore, it is recommended to reduce the reverse thrust to REV IDLE at70 kt, and keep REV IDLE until taxi speed.At taxi speed, and not above, stow the thrust reversers before leaving the runway, in order to avoid foreign object ingestion.
WHEEL BRAKES
Wheel brakes contribute the most to aircraft deceleration on the ground.
On very long runways, the use of pedal braking may be envisaged if the pilot anticipates that
braking will not be needed. To reduce brake wear, the number of brake application should be limited.

If you have a lot of runway at your disposal, I'd recommend the use of aerodynamic braking (hold a nose high attitude riding on the main gear), speed brakes and thrust reversers, followed by wheel braking starting at around 80 KIAS or so.
The reasons have to do with energy dissipation.  For an A380 landing at say 160 KIAS and following this procedure, the pilot has already dissipated 3/4 of his total kinetic energy which he started with at the time he applies the wheel brakes at 80kts, leaving only 1/4 of the original kinetic energy to be dissipated through the brakes in heat.  As drag forces are also directly proportional to the square of the velocity, it makes sense that aerodynamic braking and high drag devices will drain off a lot of energy very quickly while at high speed but their effectiveness is greatly diminished at lower speeds.
In addition, the wings are still producing a great deal of lift at touchdown, reducing the static coefficient of friction between the tires and the runway, making braking much less effective at higher speeds.
Thrust reversers are again going to be more effective at high speed as performance is boosted by ram air at the intakes and lower speed operation risks compressor stalls as well as damage from ingestion of FOD kicked up by the fan exhaust.

Holding the nose wheel up is a really bad idea for larger aircraft! From the 777 FCTM: "Do not attempt to hold the nose wheels off the runway. Holding the nose up after touchdown for aerodynamic braking is not an effective braking technique and results in high nose gear sink rates upon brake application and reduced braking effectiveness."

I experienced such a landing as a passenger and happened to film it:
A319 landing in Stuttgart (EDDS) on runway 25 (10974ft, 3345m), exited on last high speed taxiway. (Sorry for the poor video quality) Touch down at 3min 16 seconds

The pilots touched down at the normal touch down zone, used idle reverse and speedbrakes and low autobrake, then stowed the speedbrake (disables autobrake) and reverse, rolled with idle thrust and no braking down the runway at roughly 60 kts or so, then used light braking at the end of the runway to make the last high speed taxiway.


text:
Would a random passenger be allowed to fix a problem with a commercial aircraft?
I just read an article about a man (who is alledgedly an airplane mechanic) who was a passenger on an airplane that had some mechanical issues. Because there were no 'local' mechanics around, he decided to take matters into his own hands and fix the problem himself.
Here's the article I read (it's in Dutch though)
Here's one in English
This all seems nice but in this day of age where airplane security and regulation is so tight, I wonder if this is possible at all. It also seems to me that by the airplane company is taking a huge risk in letting this "random" person fix this problem. The article obviously oversimplifies a few things, but regardless of that, here's my question:
Is it possible for a random passenger, who also happens to be an airplane mechanic (working for airline X) to fix an issue on the airplane he happens to be on, given that there are no "local" mechanics around (however unlikely that might be)?

I'm pretty sure the airline did a quick reference check to see if he was what he said he was

It certainly depends on the issue. I once fixed a toilet door that couldn't be properly closed....

This almost seems more like a skeptics question than an aviation one!

Is this while the aircraft is on the ground or during flight? Would they all crash and die if this person doesn't do said repair?

I happen to have personal knowledge of this particular case. I was in that airplane, too, seated two rows away from him. I'm also a former pilot and was naturally very interested in what happened, so I asked him some questions during the flight back.
He had worked as service mechanic for this company before, servicing their planes in Iceland. Their technical staff and the pilot of this plane already knew him, which made things easier.
Tools and parts were available at the airport in Alicante, but no qualified technician certified for this type of aircraft. The technical staff of the airline contacted his boss in Iceland by phone, who agreed to sub-contract him to the airline for the duration of the repair and pay him overtime for the work.
They faxed a copy of his certification documents to Stockholm to cover the EASA regulations that require a certified professional doing the repair.

in other words it wasn't a random passenger but a qualified mechanic known to the aircrew who just happened to be a passenger on that flight. Random event, not random passenger :)

@jwenting exactly

@Thousand I'll take all the credit I can get :-) Really, it surprised me how many upvotes I got for this one. Never expected that to happen. Even beat my previous record (40) on SuperUser easily. And I get a Guru badge out of it too.

This is the best possible explanation of the reported situation, but it doesn't actually answer the more general question that was actually asked. I don't want to discredit this cool answer, but I'm still curious about the original question!

@TorbenGundtofte-Bruun I know it doesn't exactly answer the actual question. That's also why I'm so surprised about the huge number of votes. See my comment to jwentig below the answer from Emil about the general case.

I do think this answers the question. If there's the lucky coincidence of a person with enough knowledge arround, an airline may accept help from a him (as the example shows). If the plane is about to crash hollywood-like, a pilot is probably happy about any help he can get (if the person manages to convince the pilot of his skills), but as the bureaucracy needed for this case shows, it's not possibly for anyone to simply apply a fix for a non-fatal problem.

I suspect that the upvotes are due to the fact that there are 7 billion people on the planet, and the fact that one of them was both on the plane in question and posts here is so mind-numbingly infinitesimal as to be nearly incredible.

@dotancohen I signed up to aviation.SE just to commend you on your correct use of 'incredible' :)

Incredible indeed. :-)

This does answer the question: "They faxed a copy of his certification documents to Stockholm to cover the EASA regulations that require a certified professional doing the repair."

The airline Primera Air is a Danish business that was originally based in Iceland. They have a fleet of 8 Boeing 737s.
The passenger Davíð Aron Guðnason is an Icelandic air mechanic. He is reported as saying:

“I spoke to the captain on board and he put me through to the airline’s head mechanic who’s based in Stockholm, Sweden. He told me that there was a problem with the plane’s starter valve – which is relatively easy to fix. It only took me about thirty minutes to repair,” Davíð Aron explained in an interview with news website Vísir.is yesterday.

So he wasn't a random passenger, he was a passenger with appropriate qualifications who was in contact with the head mechanic of the airline's approved maintenance organisation.
To speculate a little, it seems reasonable to suppose that the maintenance organisation take legal responsibility for the repair. There is probably no legal requirement that all mechanics be direct employees of the maintenance organisation, so long as they ensure the mechanic has the required skills and qualifications. Given the airline's history it is possible that the airline's head mechanic, or one of his colleagues, knew Guðnason and/or was easily able to check with Guðnason's employer.

As I posted in my answer THERE IS a legal requirement that maintenance shall be carried out by an approved maintenance organization.

@Emil: I am suggesting the maintenance possibly was carried out by such an organisation. Either Guðnason was working for Primera's maintenance organisation temporarily or for his normal employer subcontracted by same.

Yes, I think that would have been the only way to get things moving that fast

Part M requires that:

Maintenance of large aircraft, aircraft used for commercial air transport and components thereof shall be carried out by a Part-145 approved maintenance organisation.

So, I don't really see how a company would let a passenger work on an airplane, even with a proper licence.
Not to mention the passenger would need tools to perform the job and that's not something one carries in his bag on vacation.

I have no idea what Part M and Part-145 are, but the incident apparently happened in Spain (from a quick scan, the Dutch article says it happened on the Alicante airport). Do these parts apply there?

Part M is the European regulation that lays down the requirements for the operator regarding the maintenance and airworthiness of its aircraft. Part 145 is the European regulation that lays down the requirements for maintenance organisations. According to what I found on internet the company operated under EASA certificate, so it was bound by these regulations

Would you be willing to add a link to an authoritative source for the text in question, or at least clarify in the answer (for those of us mostly just curious about the subject) that this is from European regulations?

http://eur-lex.europa.eu/legal-content/EN/TXT/?qid=1407333650492&uri=CELEX:32003R2042 points to Doc 2042/2003.

Well, if they have a proper license, it means they are an employee of (some) Part-145-approved maintenance organization.

@JanHudec: Not necessarily, the licence is not conditioned by being employed in a maintenance organization.

@JanHudec The certification is to the mechanic in person. Not depended on his employment. See my full answer below.

@emil See my onw answer. Seems to cover you doubts...

And to play devil's advocate: in a situation where there's no qualified mechanics at all and the aircraft grounded until repairs can be carried out, anyone who knows how to handle a wrench, hammer, and screwdriver is better than nobody at all, regulations be darned, they're shooting at us and we have to get this crate of the ground so fix the darn plane and we'll worry about the paperwork later :)

@jwenting It happens, especially with private/sports/bush- planes on small airfields. Commercial airlines are very leery of this though. If IATA/EASA finds out airline licenses can get revoked, pilots who knowingly condone such a repair can loose their license. And insurance companies can (and will) blacklist the airline. If I remember correctly there is an emergency clause (war-zone, natural disaster) that can be invoked to allow basically anything (including unauthorized repairs) to get passengers to safety. Subject to review later if it was justified: Ramifications will follow if not.

@Tonny exactly the kind of situation I refer to... On a regular trip, in a place safe for the passengers and crew, you wait for the company to ship out a maintenance crew (or hire one closer to you to come over) instead. That's what airlines do, and the larger ones have people on standby to do just that 24/7.


text:
How do you communicate with a military drone operator over radio?
This article (and other similar ones) states:

In remarks at the White House on Thursday, US President Donald Trump said the drone had flown to within 1,000 metres of the USS Boxer and had ignored "multiple calls to stand down".

For airplanes, ICAO standardizes 121.5 MHz and 243.0 MHz (military) as common emergency frequencies that aircraft "should" monitor when possible. If approached by an unknown aircraft, this is the frequency I'll try to raise the crew on first.
How does this work for drones? Does the same standard apply? Is it reasonable that in this case the USS Boxer's crew could expect that the drone is monitoring 121.5 (or 243.0) and that its operators will be listening?

Very interesting question !

In the same way the USS Vincennes communicated with IAF655?

@AEhere The political aspect is totally out of scope and irrelevant to my question. IAF655 was a crewed commercial aircraft; I'm specifically asking about unmanned aircraft (drones).

Also to be totally clear, I'm asking about general practices and not this specific incident.

Related

There is an obvious likely reason why Donald Trump said this, and the quotes around it indicate what the reporters think that is.

@DJClayworth I tried to be clear that I'm not remotely interested in discussing the political dimension or in answers related to same. My question is strictly about aviation and stands on its own. The background material provided is there only to provide context. I'd encourage anyone interested in the politics to visit politics.stackexchange.com.

It is unlikely that a drone will monitor 121.5 or 243.0. But making calls on those frequencies would have established the likelihood that the approaching aircraft was not an innocent manned civilian craft, and increasing the likelihood that the USS Boxer was justified in attacking it.

@TypeIA perhaps it would be be best just to remove the reference to this incident from your question? I was able to answer it without clicking the link nor having heard of this recent event, so it's probably not necessary and maybe just distracting since you are explicitly looking for a general answer.

Some UAVs carry VHF or UHF airband radios to communicate with air traffic control and other aircraft. The voice channel is relayed to and from the remote pilot operator via satellite or another form of datalink. I've heard UAVs taking with Joshua Approach in the Mojave Desert, and one time Joshua called one out to me as traffic and vice versa. I agreed to maintain visual separation, the remote pilot acknowledged and overall it was very similar to dealing with any other aircraft.
I have no idea whether they monitor 121.5 as well as whatever enroute frequency they're using, but it seems plausible that they could.


text:
How robust are modern parachutes to folding errors, wear, or other problems?
My 8 year old recently got a guided tour of the parachute-packing station at the local sport airport. Afterwards she asked me what happens if the parachute is improperly packed. Since it seemed to me that each parachute is packed by one person, there is no checking of human error.
How resilient are modern sport parachutes to packing errors? I would include in that any improper packing method or the packer overlooking some detail such as wear or tears. I'm not a parachutist so I don't know the intricacies to ask a more nuanced question.
Proud father update: A bit belated, but she's no longer eight years old and she enjoys, in her words, floating on imagination.

Not an answer: Para-Cushion Owner Manual and Ripstop fabrics.

Qualification: I am a master parachute rigger, taught the sport for 10 years and have packed a couple of thousand emergency parachutes.
I can put an apartment-size piece of nylon into a purse in under 5 minutes, and guarantee that it will open. How nicely it opens is a different question. It usually takes about 45 minutes to service an emergency parachute - 20 minutes to inspect it, 10  to fold it up nicely, another 10 to stuff it into the pack, then the paperwork. Double that time if it's a large 2-passenger parachute.
The "fold it up nicely" part is to make it go into the pack without any unsightly bulges, and the emergency packs are never a nice open bag like the main - the canopy has to be shaped around the closing apparatus and the spring-loaded extraction chute has to fit in the middle. Very little of this process has anything to do with ensuring it opens when needed.
A packer would have to be spectacularly incompetent to fold a canopy in a way that would cause it to malfunction. If you feel the fabric used to make main parachutes you will find that it is extremely slippery - good luck keeping that stuff in the proper place when it goes into the bag, but you don't need to. These things sort themselves out quite nicely most of the time.
There are two important tools used in packing emergency packs - one is appropriately called the Death Strap. It is wrapped around the middle of the canopy as you are folding it into the bag, and if you forget to remove it the canopy will not open. The other is a locking pin used to close each part of the pack in stages. Your mistake will probably make the front page of the local newspaper and involve a lot of time with your lawyer, so packers typically only own one of each and all tools are counted after each job.
Going back to the incompetent packer for a moment - the main parachute is usually packed by the owner, and very occasionally you need to open and re-close the pack. If you are lazy or rushed it is very easy to close the main pack in a way that prevents it from opening. Owners (almost) never open their emergency packs, and riggers follow a standard process to prevent these errors from happening.

Thank you paul, that is a reassuring response! How serious might an "skipped inspection" be? How much fault-tolerant damage could a parachute sustain that might be missed while packing it up, if the chute had not undergone a proper inspection?

@dotancohen If you mean mechanical damage, if your little finger doesn't go through it, it usually doesn't get fixed. Canopies can work just fine with holes your child could fit through (but we do fix them). If you mean packing errors, it really depends on exactly what happened.

Beautiful answer paul, now with the edit. I've also got some terms to Google for. Thank you very much.

maybe you could add something about why sometimes chutes don't open.

@rbp User error is pretty common. Bad body position upon deployment can cause it to wrap itself up, or delay opening long enough that you run out of altitude.

"running out of altitude" that is a phrase I need to remember.

@rbp Those ones lack the guarantee Paul provides. :)

Paul, I'd like to take the time to say an extra thanks. I don't know how much your answer inspired her, but she's been bugging me to take her skydiving since, and we just did so for her tenth birthday. Video link in OP. Thank you!

Absolutely amazing post, thanks.  Regarding the "counting tools" - just like gut surgeons.

Pauls answer is good but I think there is a few parst missing.
What could you do to make a parachute not open?
A lot.
But most of the things are easily catched.
You could flip the parachute through it's lines.
That usually happens when you have landed and pick up the parachute, you put your hand at the wrong place and manage to make a wrap of some kind.
It happens easily in rushed moments or windy conditions.
How do you notice the problem?
As you pack the parachute the lines should go straight from the "backpack" (rig) and the fabric, if it intersects somewhere you got a problem.
A line could wrap around the fabric.
This is hard to do while packing and not noticing.
You could create a bag lock.
All the fabric is packed in to a nylon bag closed with rubberbands (or magnets on newer models).
For the sake of simplicity, if a line intesects another line inside the rubberbands then it could lock.
Again it's hard to not notice. But it does happen.
You could create a pack lock.
That is when you wrap the bridle the wrong way behind the closing flaps.
The result is that it can't open however much you pull (unless you pull to destruction).
All these errors can happen but they are all easily spotted.
Most of the time you don't even look for the errors when you pack a parachute, you just feel that something is not right.
But new skydivers put lots of time and effort in to looking for errors, and have checkpoints where they call for inspection of a seasoned skydiver to have a look.
When you sum it all to one sentence:
They are not resilient, but because there are a few "rules" you pack by then you need not follow them or be very sloppy to fail.


text:
Did Clement Ader make the first flight even before the Wright brothers?
Clement Ader invented in the late 1890s, a plane that looked like a bat. It had a steam engine. Witnesses said it went airborne at least once and recorded a distance just over 50 meters. The Wright brothers made their first controlled flight in 1903.

See also this question:http://skeptics.stackexchange.com/questions/4661/were-the-wright-brothers-the-first-to-fly

John Brown, an Australian aviator seems to have proved that Gustave Whitehead, who emigrated in the US, but was born in Germany as Gustav Weisskopf, flew a powered aircraft in 1901. Brown has set a site with lots of facts. On the other hand, the Smithsonian is bound to state the Kitty Hawk flight was the first one regardless of the truth, else the Flyer would be back to the Wright family...

Lots of people flew before the Wright Brothers.  Some of them even survived!
The Wright Brothers are credited as inventors of the airplane not because they flew, but because they were the first to take flight in a vehicle which:

Lifted itself using its own power (which gliders and kites did not)
Was aerodynamically controllable by the pilot (which Ader's aircraft was not)
Remained in sustained flight, rather than taking momentary flight before returning to Earth

Don't forget that there was a lively discussion before Orville gave the 1903 Flyer airplane to the Smithsonian. He had lended it to the British Museum before, precisely because the Smithsonian would not accept his condition that the Wright brothers must be named as the sole inventor of the airplane (which they weren't - it was a collective effort). After WWII the Smithsonian changed its mind and accepted the condition.

@PeterKämpf: Indeed, the aerodynamicist (forgot his name) who the Wright Brothers consulted when researching flight got really pissed off that the Wright Brothers attempted to patent the airplane. Before the Wright Brothers, advances in flight were considered academic (what we'd today call open source). The Wright brothers didn't succeed in patenting the airplane, only the flex wing mechanism that gave them roll control. Ailerons were invented partly as a work-around for the patent.

@slebetman: Actually, both wing warping and ailerons had been invented before. I consider the Wright brothers the first patent trolls in aviation. After all, they did not design new airplanes after the Flyer III, but tried to milk their patents.

Don't forget Santos Dumont, the first to be able to do it with witness and able reproduce it anytime https://en.wikipedia.org/wiki/Alberto_Santos-Dumont#The_first_fixed-wing_aircraft:_The_14-bis_versus_the_Wright_Flyers

@PeterKämpf: wow, the draft of history is negotiable...!

@jean : I don't know if you're just kidding but the wikipedia page you cite mentions "official witnesses" as opposed to just plain witnesses. There were five witnesses present at Kill Devil Hills on Dec 17 1903 for the Wrights first flight, albeit not officers of the Fédération Aéronautique Internationale but as that body didn't exist until almost two years later it would have been rather difficult to arrange.

@PeterKämpf You forgot to mention another part of the Smithsonian dispute. Samuel P. Langley, secretary of the Smithsonian. His own attempts at powered flight using his aircraft 'Aerodrome' were complete failures and yet the 'Aerodrome' was displayed for a number of years in the Smithsonian as the first powered heavier-than-air aircraft. It was not too surprising then that Orville Wright insisted that if their craft was to be displayed it should given its proper priority. More details of this bizarre story here : https://en.wikipedia.org/wiki/Wright_brothers#Smithsonian_feud .

In a way, he did. But he had little control about all axes. Derisory comments even suggested he only took to the air thanks to his bumpy runway.
But there were plenty more before him. The first flying objects which anticipated the airplane were the models of Alphonse Pénaud. He finished the Planophore in 1871 and influenced all who came after him. Unfortunately, at his time there was no energy source to power a man-carrying aircraft, and his models used rubber bands for storing energy. Please note that he did not forget to add a vertical tail; his design did not need it - the tail-mounted propeller stabilized it well enough.
The next big step was made by Otto Lilienthal, and again the lacking power source restricted him, in his case to man-carrying gliders. He saw the airplane as a piece of sporting equipment, like a racing bicycle, and priced them accordingly. He sold over a dozen of them to people all over the world. And he was the first to use systematic scientific research: He found out that a thick, blunt leading edge and wing camber help to improve L/D. The polar plot has its name from him. He experimented with carbon acid engines, but never used a propeller, flapping the wing tips of one of his designs instead. He also invented wing twisting long before the Wrights, and shared his results freely with the aviation pioneers of his time. The Wright Brothers built their success on the basis of his work, and were much more secretive.
If we mention Clement Ader, we also need to include Hiram Maxim and maybe even Samuel P. Langley. They (and a few more) all tried to create powered aircraft, but did not get it all right. At least, Langley's assistant Manly and S. Balzer gave the world the radial engine.
The next pioneer would be Gustav Weißkopf (or Gustave Whitehead) who seemed to have flown a plane which took a lot of inspiration from Lilienthal's designs, but added a boat-like fuselage and wheels, so he had his hands free for steering the plane. Lilienthal had put himself into a tight corner by foot-launching his gliders and needing his hands to hold them. Whitehead flew in 1901, but discontinued his work to focus on his family. But he was the prime address of his time for airplane engines; many of the early pioneers bought his engines.
Only now the Wrights contributed their part, and are known as the inventors of the airplane due to a contract Orville made with the Smithsonian: The Smithsonian got the 1903 Flyer, but needs to present the Wrights as the sole inventors of the aircraft. Orville knew exactly how weak this claim was, so he insisted on this clause.
Meanwhile, in Europe, a number of pioneers achieved also powered, controlled flight without using the Wright's knowledge, because Orville and Wilbur kept their findings under tight wraps and started suing everyone who would also attempt to take to the air. But at the Le Mans flight meeting of 1908 the Wrights showed off their more maneuverable design, shaming all European designers. How little lasting effect they had in the end becomes clear when you look at the airplanes that followed: The inspiration clearly came from others.

It is seriously disputed that Whitehead actually flew in 1901.

Ader's maximum altitude was a few inches.

If you're counting gliders among the precursors, "The first flying objects ... were the models of Alphonse Pénaud " would have come as a surprise to George Cayley (who did employ a vertical tail fin).

What Cayley did is hard to verify - there is much less evidence than for the flights of Whitehead, and Whiteheads achievements are mostly ignored for that reason. One granddaughter and one coachman made some remarks decades after the fact - if he really had been successful, would he not had continued and would not had others tried to follow suit? Please look at the planophore, and then at the sketches we have from Cayley: Which one really looks like an airplane?

The 1897 plane of Clement Ader has never left the ground,
see: General Mensier's Report
Clement Ader did not fly. The October 21, 1897 report made by general MENSIER and its annex from October 27, 1897 (see the link) are clear, Ader’s plane never left the ground with its front wheels in both the October 12 and 14, 1897 officially witnessed tests. The October 14, 1897 alleged take off and short flight is a pure myth contradicted by historical documents.
Was Clement Ader an aviation pioneer? Yes, because he built a plane in 1897. Was he a flyer? No. Did he fly earlier in 1890? No. The 1897 (Avion III) plane was seen on the ground by General Mensier and his comision. Other earlier planes (Avion I and II), Ader claimed he had built, has never been seen in the air or on the ground.

The Langley machine was a disaster, despite its advanced Manly radial engine. Langley had given almost no serious thought to the problems of stability and control. But the Smithsonian shot itself in the foot when it hired Glenn Curtiss to "make a few adjustments" in order to make that machine fly. And Curtiss was indeed sorta successful a decade later, giving rise to the Smithsonian's claim of priority (and a "successful" use of the $50K of taxpayer money Langley had spent!)
It was this skullduggery which enraged Orville Wright and led to his shipping his Flyer to London. It was only after WWII that the Smithsonian mended its ways with Orville, documenting all the extensive modifications Curtiss had made to the Langley machine, that Orville consented to the return of the 1903 Flyer.

Montgolfier brothers were the first to invent a flying machine...
But if you are looking for piloted, controlled, heavier-than-air, sustained, flight the Wrights were the first one to do that (their longest flight on December 17, 1903, was 852 feet). Ader didn't really have control or sustained flight. Objections that the Wrights tried to keep their invention under wraps are noted--and sustained. Early on, Santos-Dumont did much more for the science and art of flying in the mind of the public, but he was not the first. Whitehead spun some great yarns but never flew a powered airplane (as vociferous as Whitehead's boosters are, the proof for any powered flights is thin and poorly substantiated).

Fact: Man kind only flew in 1906. That glider wasn't able to fly. Check this article
US patent was only granted the "invention" to the brothers many years later on a new "re-invented version" with the purpose of allowing airplane to be produced for military purposes.
Now check the video of first airplane really flying, greatest moment for human kind can't be stolen by fake papers:

Welcome to aviation.SE. Please consider adjusting your language, "fact:" and "fake papers" sound a bit like a conspiracy rant, and we are not too appreciative of those. Moreover, I don't see how that wired article would support any of your statements, the fact that a replica would not fly is not proof that the original could not either.


text:
Aerospace Engineer program in EU?
I am looking for an Aerospace Engineering program in the EU. I heard the Aerospace Engineering Department in Delft University in the Netherlands is one of the best programmes.
I prefer English speaking countries, such as the Netherlands, Scotland, and Scandinavian countries.
To be more specific I would like to study about the operation of airlines, airports, maintenance, how to do aircraft take-off performance calculations, and regulations relating to the operation of EFB applications.

Perhaps you can make your question more specific, as it is now it is a little hard to answer.

You are aware that your question is biased? If you want an unbiased answer, you should probably leave out hear-say information. And you should inform yourself on the curricula of different programs. Some of your topic requests are very specific, and will more likely be available as specialization/MBA modules later on (if at all, some of them sound more like logistics than engineering).

Updated: February 2016
Keep in mind this is aerospace engineering, but some places offer stuff like aviation management, which might be more what you are after, depending on what you want to do. TU Delft is more tailored to the former.
While most of the topics of interest you mention will only appear in some capacity in the TU Delft AE BSc, they will appear in more detail and focus in certain MSc tracks. I'm not sure which you are after.
While you mentioned Sweden, they do the first year BSc in Swedish, which stops you unless you know the language (All MSc is English). KTH (Kungliga Tekniska Högskolan), Stockholm is normally considered the best TU in Sweden and does AE as MSc. Again, it might not match what you're looking for. Scotland has plenty of programs- a search in the UCAS serach engine should suffice.
Wherever you go, it's a big cultural question as well. What are your expectations of for instance social life? There are two similar but still different countries & cultures.

I am a former BSc Aerospace Engineering student at TU Delft. In my opinion, the uni has great facilities and it's for most part a very good (of course with the occasional bad lemons that you would find anywhere). Student teams are also of very high quality. The topics you mention thought would probably be more dealt with in the master program however. The BSc. is pretty 'heavy' engineering (calculations/exercises) for most part with a lot of conceptual and a little practical work to try and brighten up your day :) TU Delft may be good but you won't get through it on a banana peel. First year dropout rate is ~55% according to one source. University officially says 76% however. A minority pull it off in the official three years but most take longer. Important consideration if you’re paying the non-EU fee.

The topics you describe are certainly part of aerospace engineering, however they are somewhat specialized topics. I am not trying to discourage you, but Aerospace engineering involves a lot of mathematics and physics. If these fields do not appeal to you, you will be having a hard time to get through to the sweeter parts.
Some universities only offer a Master programme for Aerospace engineering, which typically follows on to their Bachelor programme Mechanical engineering. If you are really interested in aviation, I would recommend to select a full (Bachelor and Master) Aerospace programme.
Aerospace engineering programmes generally involve a thorough basis of mathematics (linear algebra, calculus) , physics (mechanics, thermodynamics), material sciences before introducing more specialized subjects like aerodynamics, aircraft performance, and space flight. At bachelor level, you will get an introduction to a diverse set of aerospace related subjects. At master level you will go deeper into an area, although you will have to accept that you won't master all subjects you mention.
I had a good time at Delft University, it certainly has a good Aerospace Engineering programme which I would recommend.
In addition I had the pleasure to work with a graduate from Bristol university, she is very knowledgeable. I'd certainly look into their programme as well.
For Scandinavia I would look into aerospace programmes in Sweden. Sweden has quite an active Aerospace industry. Linköping is their centre of Aerospace Engineering.


text:
How is the weight of an airplane measured?
Many very large airplanes have specific information of their empty weight. For instance the weight of an empty, regular 747-400 is 393,263 lb (178,756 kg) according to Wikipedia. The Concorde is 78,700 kg.
How is the weight of these airplanes measured so precisely (note that in the 747-400 example is precise to the unit's place!). Is the sum of the weights of all the parts used? Also, is Zero Fuel Weight (ZFW) the same as empty weight?

The 747-400 example is precise to the unit's place; that doesn't mean it's correct (accurate).

The empty weight of airplanes can only be given to single-pound precision for one specific airplane. Minute differences in metal sheet thickness, equipment and paint will make sure that there is a difference of many pounds between airplanes of the same type and furnishing. Add to this the difference in seating, galleys and instrumentation between airplanes of the same type which are operated by different airlines, and a precise number as in your question becomes almost meaningless. A better way would be to give a range in which most airplanes will fall, or to round the number like the Concorde weight of your question.
Weighing is quite straightforward: All tires of the landing gear are placed on scales and the weights are summed up. If the attitude of the airplane does not change, the weight on the tires can even be measured consecutively, so only one scale is needed. Small floatplanes can be suspended from a crane with a scale in the line holding the aircraft.
In Ernst Heinkel's biography he remembers an aircraft deal with the Soviet Union in the late 1920s. He had to deliver five floatplanes of a specified weight, and the Russian approval inspectors insisted on all planes having the exact same weight. Of course the planes were not identically heavy, but his advantage was that the scale in his factory was indicating the weight by printing it on cards. So the evening before the inspectors came, he let one aircraft be weighed five times, put the cards back into the machine and had the printing mechanism disabled. The next day he closed the business successfully with his very happy Russian guests.

By "advantage" do you mean fraud?

@raptortech97: Yes, technically this was fraud. Maybe a comparison can be found in the software industry, which is expected to sell flawless, perfect products. This is just as impossible as airplanes of identical weight.

@PeterKämpf: Why could one not simply identify the heaviest airplane, and add a suitable amount of ballast to each of the others to make it match?

@supercat: That would work, but is an ugly kludge. The pride of a plane maker would be incompatible with that.

@PeterKämpf: Why would it be less compatible with that than with jinxing a scale? If the maker was interested in pride, it could use weigh each plane a few times using multiple scales simultaneously, determine for each plane the center of mass and the moment of inertia about each plane, and add ballast to precisely match not only the total weight but also the its effective distribution.

That's amazing - they have crane scales to weigh float planes? They must be enormous. Is it like the scales at the grocery stores that you weigh your produce in? (except slightly bigger of course)

@coburne: Sure! Just look here. How else would you weigh a 6-engine flying boat with 46m wingspan?

Addressing the ZFW question, zero fuel weight is not the same as empty weight.  Empty weight is the weight of the airplane without any people, bags, cargo or fuel on board.  Zero fuel weight is the weight of the airplane plus people, bags and cargo, but does not include fuel.  The ZFW is important because there is usually a limitation of maximum ZFW. For example, once you hit max ZFW you can no longer add people or cargo to the airplane, even if you would stay under max gross weight including fuel.  The only weight you can add to the airplane above the max ZFW is fuel.

For the pilots operating the aircraft, the ZFW is useful because one needs to determine the weight of the aircraft at various points in flight, and the easy way to do that is to add the current fuel (or the projected fuel at the point you're interested in -- such as landing) to the ZFW.  A modern FMC can tell you the current weight, but to determine weight at landing, you typically have to do the math yourself

You get yourself some load cells, put them under the gear axles / lift points etc. and jack it off the ground. The attached computer will tell you the total mass, center of gravity etc.
Several companies produce aircraft weighing scales.

I agree.  I have watched them roll planes (G5 size) onto load plates and gather up the values.  They did this after major maintenance that effected the weight, like rewiring.  Then the W&B paperwork was updated with the new values.

Be aware that if a small change is made to an aircraft, like adding or removing equipment, the airplane is typically not reweighed but it's empty weight is recalculated by adding or subtracting the weight change. Also, the empty weight c.g. is recalculated by adding or subtracting the moment change.
As I remember, there is also an requirement that aircraft be reweighed every so many years. I want to say every 3 years, but that may be wrong.
Also, the empty weight does not include pilots and cabin crew. The term BOW (Basic Operating Weight) is often used that does include standard weights for pilots and cabin crew.
Air carriers, particularly freight operators, may employ more than one BOW to allow them to easily switch between configurations. For example, freighters sometimes carry fly-away-kits (FAK) of several hundred pounds. The kits are strapped down, but can be quickly removed if need be. The operator might choose to have two BOWs, one including the FAK and one without it.
There's also the matter of using "fleet weights" rather than individual airplane weights, or at least there used to be.

in my line, whenever we exceed a change of more than 1% BOW, the W&B needs to be updated.  not sure if this is industry-wide or contract-specific.


text:
Are four-engined airplanes subject to ETOPS equivalent rules?
I've heard of ETOPS, which permits the maximum duration for which a two-engined airplane can fly on one engine to the nearest airport.
Are 4-engined planes such as the Boeing 747 and Airbus A380 subject to any equivalent rules? If not, can these planes safely fly on just one engine for as long as needed?
Also, how are aircraft given different ETOPS certifications? What makes the 787 get ETOPS-330 and the 767 only ETOPS-180 despite the similarity in range and (to an extent) engine power and design?

787 is ETOPS-330 since 28 May 2014. These certifications can only be obtained after the type flew certain number of hours and demonstrated certain reliability.

Ah, I edited my question. Thanks @JanHudec!

747 and A380 are not designed to be flown on one engine only.  They can, however, fly when one or two engines are down (two or three engines still running).

Yes, three- and four-engine airliners follow 'Long-Range Operations' restrictions just like twin-engine airliners.
From WikiPedia:

Airbus proposed to replace ETOPS by a newer system, referred to as LROPS or Long Range Operational Performance Standards, which would affect all civil airliners, not just those with a twin-engine configuration
(...)
The FAA has decided to use the single term, 'extended operations,' or ETOPS, for all affected operations regardless of the number of engines on the airplane.

The large 4-engined airliners can not reliably fly long distances on one engine only.  But  they can fly on three engines just fine.  When only two engines are available they divert to the nearest airport.
The ETOPS rules become relevant whenever a three- or four-engined jet diverts more than 180 minutes from the nearest airport.
For more information, refer to the airliners.net thread on 3 And 4 Engine Planes Requiring Etops.

Note that while Airbus proposed the name change to LROPS, that didn't actually happen. I'm fairly certain the FAA still calls it ETOPS and I think ICAO does, too.

Other way round - the multi-engine rules existed long before ETOPS.
And no 747 pilot will continue as scheduled on one engine. If you are having enough problems that you lose 75% of your power you will punch "Nearest" on the navi and go there.

But I suppose "nearest" won't be an option if you're in the middle of the Pacific or doing a transpolar crossing.

It's always an option. A 747 (or any other certified aircraft) can continue cruise flight on one engine, but not well. The crew will head straight for "Nearest", dump excess fuel, make a straight-in approach channeling Zen, the Force and calling in any deity's favours they may have. No go-arounds.

A British Airways flight lost an engine just after takeoff from LAX in 2005 and continued all the way to the UK: https://en.wikipedia.org/wiki/British_Airways_Flight_268

Also, while loss of an engine is an emergency, it is not such an emergency that would preclude a go around.  There isn't anything terribly urgent about flying on one engine in the absence of something more serious like fire, smoke, some sort of other damage.

@Ralgha Flying on one engine in a 747 (which means you've lost three engines) would indeed probably preclude a go-around, due to lack of available power. Unless, of course, we're talking about this 747.

@reirab those test flight pics show the amazing changes in the airline industry in just 30-40 years. Sadly, low clouds earlier this week cancelled a 747 fly-by showing off the new RR Trent 1000 TEN

@FreeMan That stinks about the fly-by being cancelled. But, yeah, the GE90 is pretty much a beast. Once airborne, GE shut down the other three engines and flew that 747-200 on just the one GE90. :) It's amazing to have an engine whose nacelle diameter is about the same as the fuselage diameter of a 737.


text:
Where can Boeing wind data be found?
There are various services that use world-wide Boeing Winds for forecast wind data which can be used to calculate an approximate flight time between two locations.  They usually have best case, worst case, and average case for each location, altitude, and date in the future.
I have searched and searched Google to no avail.  Where can this wind data be found, and how can it be used in a commercial product?
For those of you who don't know what the Boeing winds are, I found this description of their software product on am informal message board (not related to Boeing):

PC WindTemp
INTRODUCTION
  The Boeing PC Windtemp program provides global average and statistical winds and temperatures aloft over a 30 year
  period. These data are used primarily for comparative transport
  airplane performance and economic studies. The data may also be used
  for transport airplane route planning on an annual, seasonal, or
  monthly basis.
Note: Actual winds and temperatures at a specific location and time
  may differ appreciably from those computed by PC Windtemp. Therefore,
  data from PC Windtemp must not be used for flight planning, aircraft
  dispatch, airborne flight navigation, or any other type of actual
  flight operations.
PROGRAM FEATURES
  A seasonal and monthly database with global coverage from 1000 millibars (364 feet pressure altitude) to 10
  millibars (101,885 feet pressure altitude). Annual data updates to the
  most recent 30 year period. Airport codes and coordinates may be
  imported from a text file in a comma separated value (CSV)
  format Calculates enroute wind and temperature for up to 50 airport
  pairs at a time. Calculates enroute wind and temperature for simple
  great circle tracks or tracks with up to 250 waypoints, with or
  without altitude changes. Calculates average enroute wind and
  temperature or any reliability (quantile) between 50% and 99%. Months
  or seasons, reliabilities, altitudes, and speeds may be nested for
  input efficiency. Formatted and text output reports are
  available. Program subroutines (DLLs) can be called from other
  programs written with C++ or Visual Basic in order to incorporate
  Windtemp functionality in those programs.

Are you asking about getting Boeing's data specifically, or are you asking where someone can get equivalent data, even from another source?

@Pondlife Either would be good, but I am still curious about Boeing!

An alternative to the Boeing data (if it exists, you might need to call one of their sales reps to find it) is publicly available meteorological data.  One such project is the NCEP/NCAR Reanalysis Project.  There are a few others, as well as archived data.  None of these is exactly the Boeing dataset, but I'd bet that the Boeing dataset is based at least in part upon data like these.  The benefit to the data below is that in most cases it is free and there is a lot of it.
Reanalysis Data
Reanalysis data is constructed from the best available past observations, which are assimilated into a weather model to fill in the gaps between obs and in time.  There are many such projects with various scales (global, regional) and time periods.
The NCEP/NCAR Reanalysis Project
This dataset has 4-times-daily, daily and monthly values between 1948 and present and long term monthly means for 1981-2010.  The data is global. The data is publicly available and you can retrieve files in NetCDF format from the linked website.  The wind data is stored on pressure levels (e.g. 1000 mb, 500 mb, 250 mb) and is stored in its u (east-west) and v (north-south) components separately.
The ERA40
This dataset is a global dataset covering 1957-2002 and has various output resolutions available.  Output from ECMWF is typically in the GRIB format and not all of it is free for non-research use.
The NARR
The North American Regional Reanalysis, as the name suggests, is a regional dataset covering North America.  The data covers 1979 - present.
Observations
Long term past observations are archived at the NCDC and include surface stations, weather balloons and model data, among others.  From the various models available a statistical representation of winds could feasibly be produced.

The wind data that Jeppesen/Boeing uses is proprietary, they have their own meteorologists.
Similar data is available from NOAA.  They have a data server that can provide wind information.

Does the NOAA also provide historic data, or only current data?

Thank you, but I am not looking for current data like NOAA offers.

NOAA historcal data is here:
http://www.ncdc.noaa.gov/

Usually inside the dispatch document, you will have a list with all enroute fixes, like this:
FIX    FREQ     AWY   WIND    COORDS
---------------------------------------
VKZ    123.45   UN857 235/60º W... S...

Usually airlines have their own system to deal with that, but search about lufthansa's LIDO, they can provide this information for you, click here to understand a little bit more.
And as said by Jim, all data is provided by NOAA via winds aloft charts, you can find it here, section "Wind and Temperature".

Thank you, but I am not looking for current data like NIAA offers.

I needed to draw 85% annual wind range maps and I could not find any such tool so I wrote the software using all meteo data listed above.
Have a look at http://aircraft-range.com
It is free and includes over 5000 airports and 500 aircraft.
You can also create/ edit aircraft.

Nice info, but the OP asks about Boeing data, so this doesn't seem to answer the question.


text:
What is a flight leg and why it is so important for an aircraft?
A flight leg is basically flight from one point to another point, (I'm not sure if flight leg number increase with touch and go around). I would like to know more about flight leg under what all conditions usually it increases and when it is reset.
Also, most important why is it so important for an aircraft/airlines because usually it is 1 or 2 flight leg per travel. From what I know, flight leg is most important parameter in case of recording or storing data in NVM but I believe flight phase should be sufficient enough.

What do you mean by "NVM"? This TLA does not seem to have any ASM.

@JanHudec NVM = Non-Volatile Memory; it's a computer/avionics term referring to memory where configuration, settings, maintenance and other data can be stored persistently.

If I understand it right, a flight leg is just one part of a flight cycle - the time between take off and landing. Correct? Therefore the number of flight legs shall be equal to the number of flight cycles during the lifetime of an A/C.

With respect to avionics and fault logs, flight legs is just one more data point. It's not any more important than any other piece of data, but it does have value. Avionics are not structure and don't care as much about pressurization cycles, but it can be a factor - the pressure change between sea level and 8000 cabin altitude does have some effect. It's just that avionics can easily count flight legs and it makes for an easy way to organize the fault logs along with flight phase.
It is useful data when trying to find the root cause of an intermittent fault in a piece of avionics.  Fault logs typically log at least 20 legs before they "roll over" and overwrite old data.  With some aircraft flying 6 or more legs a day, that ensures the plane can get to a base with maintenance before the fault gets overwritten.
The value is the ability to help detect patterns.  I've seen faults that only occur on the first leg of the day after the plane was parked overnight at a very humid airport. Re-creating the conditions allowed us to duplicate the fault. The problem was a flaw in the conformal coating (sealant) on a circuit card that allowed moisture to cause a short.  Once the unit had operated for an hour or so, the heat had dried it out and it worked fine.

I don't think flight leg is important for either aircraft no airline.
The reasons why most flights have one or two legs is purely economic. Aircraft have to be operated with rather high load factor to be profitable. If you have a line with less passengers, you really want to use smaller plane or run it less often. Because different links have different demand, it's rather rare to have many consecutive links with similar demand so it would make sense to fly them with one plane as multi-leg flight. And few people would want to fly the whole trip as the stop-overs take rather long.
For aircraft what matters is not legs, but cycles, meaning pressurization cycles. For pressurized aircraft the material fatigue is most closely related to the cycles and since each cycle also involves one period of running the engines at high thrust, the engine wear is also somewhat related. It does not matter whether the aircraft accumulates 4 cycles in 4-leg journey or 4 separate journeys.

Yes from Material Fatigue point of view, aircraft maintenance are more concerned about pressurization cycles.
But from other point of view like investigation, analysis etc would like to know flight leg importance.
For example primary flight control computer (part of Flight Control System) while storing faults in memory it stores date, time, flight no., flight phase, flight leg etc. And also special importance is to flight leg, whenever flight leg increase base address of memory is increased for fault storage so that faults can be grouped per flight leg.

what is flight leg?

A flight leg is a trip of an aircraft, from take-off to landing.
Use of the term implies a multi-leg flight.
From the point of view of an aircraft's operator, it is a smaller part of an overall journey which involves landing at an intermediate airport. Each leg starts and ends at a different airport.
From the point of view of a passenger it is a part of a journey that may start or end with a stopover, a change of plane or a change of airline.

why is it important for an aircraft

It isn't really. See Jan's answer regarding aircraft cycles.
It is obviously important administratively for scheduling and financially for optimizing return on investment etc.